Chemistry 2720 Fall 1999 Test 1 Solutions

1. 1. False. The first law only forbids energy nonconservation. Melting ice below (forming supercooled water) is allowed by the first law. It is only disallowed by the second law.
   2. True
   3. True
   4. True
   5. False. The enthalpy of formation of a hydrogen ion is arbitrarily set to be zero, which makes the enthalpies of formation of other aqueous ions measurable.
   6. True
   7. False. The of a reaction in and of itself tells us nothing about whether the reaction is spontaneous. To decide whether a reaction is spontaneous or not, we need to know for the Universe (or at least for an approximately adiabatic subset thereof) and not just for a reaction.
2. at constant pressure. In order for the pressure to be constant, the external and system pressures must be identical. The work at constant pressure P is . Therefore, . Substituting this relation into , we get .
3. If the maximum safe temperature rise is 6K and the heat capacity is 10kJ/K, the maximum energy release is

   

   Coal releases 25kJ/g, so

   

4. 1. During a reversible expansion, . Therefore . For an ideal gas, PV = nRT so we have

      

   2. No process can produce more work than an equivalent reversible process. The claim that the machine produces 3.5kJ/mol is in error. Your friend shouldn't invest.
5. We start by calculating the enthalpy of reaction at :

   

   The enthalpy of reaction is related to the enthalpies of reaction by

   

6. We want the entropy of liquid potassium at 1000K and we have the entropy of solid potassium at 298.15K. We imagine the process

   

   We just need to calculate the entropy change for each of the steps and add this to the entropy of the solid at 298.15K to get the desired entropy.